The burning of cement clinker is currently carried out almost exclusively in cylindrical rotary kiln installations at temperatures of around 1,450° C. The energy required for burning is generally provided by burning conventional fuels, such as coal, in the main burner. For cost reasons, the conventional fuels are being increasingly replaced in practice by replacement fuels. However, the use of replacement fuels leads to a change in the flame shape and the temperature distribution in the kiln and thereby to an increased energy requirement, which results in correspondingly higher exhaust gas temperatures.
The cement raw meal is heated and calcined in a heat exchanger which is usually constructed as a counter-current cyclone heat exchanger with the kiln exhaust gas before being introduced into the kiln. In the heat exchanger tower, there is generally a calcinator upstream of the kiln inlet. In addition to the main burner, fuel is fed in the calcinator or at the kiln inlet, whereby a high calcination rate is achieved, the volume flow in the kiln is reduced and the clinker production is increased. It is also possible to use as a fuel in the calcinator inter alia biological waste, such as sewage sludge.
The sewage sludge of the communal waste water purification operation typically has, after mechanical dehydration, water contents of approximately 70% by weight. However, the introduction of sludge with high levels of moisture and correspondingly low thermal values would make no significant contribution to the calcination. Instead, the introduction of sewage sludge with high levels of moisture can be considered as disposal of the sewage sludge. At the high temperatures in the calcinator, odorous substances, such as carbon and ammonia compounds, become decomposed. However, the introduction leads to an increasing volume flow of exhaust gas, which requires greater sizing of the preheater or limits the production in existing installations.
It is therefore advantageous to dry the sewage sludge before it is added. The exhaust gases which are produced during drying (vapour or air) are loaded with carbon- and ammonia-containing odorous substances, the preparation of the loaded exhaust gases or the exhaust vapour condensate (after condensation of the exhaust vapour) being able to be carried out by means of washers and/or biofilters. However, the complexity of the installation increases significantly as a result. It is therefore proposed in WO-2010-124702 to guide the exhaust gas which is produced into the clinker cooler whereby, however, a reduction of the cooler output must be taken into account. US-2005-0274067 describes the supply to the cement production process, but with the supply to the calcinator or the kiln leading to lower clinker production.
In EP1930303A1, raw biogas is used as a fuel in cement production, the raw biogas containing significant amounts of ammonia, which act as a reduction agent with respect to nitrogen oxides.